The magnetic field distribution between substrate and target in a dc magnetron sputtering system was altered by superimposing an axially symmetric external magnetic field B(ext) to the original field of the magnetron source. The magnetron was operated with a Mo target and Ne, Ar, and Kr discharges were investigated. The plasma parameters in the substrate vicinity were measured as a function of Be, using both cylindrical and flat Langmuir probes. It was found that the variation of the magnetic field strength caused large changes in the plasma parameters, such as charge carrier density, plasma and floating potentials, electron temperature, and electron energy distribution function while the cathode discharge processes remained unchanged. The Mo deposition flux was found to be independent of B(ext), but the ion saturation current to the negatively biased substrate holder could be changed by a factor of approximately 50 by varying B(ext). In addition to the ion flux, the two other sources of energetic species arriving at the substrate-the sputtered Mo atoms and the reflected Ne, Ar, and Kr neutrals from the target were calculated using a TRIM computer code and taking into account gas phase scattering en route between target and the substrate.